It is of interest to CBER whether cell lines used for product production contain virus or virus-like particles or viral nucleic acid sequences or support the growth of viruses infectious for humans. In previous studies, we used electron microscopy to examine endogenous retroviral- like particles found in the Chinese Hamster Ovary (CHO) cell line which is used as the cell substrate for several recombinant products made in CHO cells. CHO cells contain endogenous retroviral-like particles visible by electron microscopy. We attempted to determine whether growth factors or cytokines (which might be manufactured in CHO cells) might affect these endogenous particles. The results indicated increases in particles from 30-100% after treatment of CHO cells with several cytokines (GM-CSF, IFN, TNF) or with the phorbol ester TPA. A manuscript describing these findings has been published in In Vitro Toxicology (1991). We have recently approached another cell substrate issue. Since many new recombinant biological products are produced using the Baculovirus expression system and use an insect cell line called Sf9; and relatively little is known about other viruses which can infect Sf9 cells, particularly those with human host range, we recently initiated a study to determine whether any viruses pathogenic for humans can infect Sf9 cells. We have examined a series of viruses for their ability to infect Sf9 cells, and protocols for evaluating whether Sf9 cells are free of viral contamination and suitable for use for production of biological products have been developed. We evaluated the ability of Sf9 cells to be infected by fifteen viruses chosen to represent several classes of arboviruses as well as several additional viruses with wide host range. St Louis Encephalitis virus (SLE) a flavivirus, can productively infect the SF9 cells. Results with three other flavivirus, attenuated strains of Yellow Fever (17D vaccine strain) (YF), Dengue virus 1 (DEN-1 and 2 (DEN-2), suggest possible low level infection which cannot be sustained. SLE causes CPE in the Sf9 cells (10-20% giant cells). YF, DEN-1, and DEN-2 do not produce obvious CPE in the Sf9 cells. SLE can also be detected in SF9 cells by immunofluorescence and electron microscopy. Plaque assays on Vero cells using filtered supernatants from SLE-infected Sf9 cells are positive. Furthermore, many of the viruses we tested apparently can survive for long periods in cell culture without causing apparent CPE, several can survive up to 8 weeks in media alone. This may present risks of contamination for products produced in Sf9 cells if appropriate testing is not instituted.